Broadband klystron



Sept 17, 1963 P. w. CRAPUCHETTES ETAL 3,104,340

BROADBAND KLYSTRON Filed Oct. 15, 1959 s Sheets-Sheet 1 p 1963 P. w. CRAPUCHETTES ETAL 3,104,340

BROADBAND KLYSTRON Filed Oct. 15, 1959 3 Sheets-Sheet 2 United States Patent Oil" assist atented, Sept. 17, 1963 3,104,340 ERGADBANE) KLYSTRGN Paul W. Crapuchettes, Atherton, and Joseph F. Hail, Redwood City, Calif., assignors, by mesne assignments, to Litton Eieetron Tube Corporation, San Carlos, Qalifi, a corporation of Delaware Filed Get. 15, 1959, Ser. No. 845,728 11 Claims. (Si. 315-539) This invention relates to a broadband kylstron vacuum tube, and more particularly to a multi-cavity klystron which employs a form of folded Waveguide structure for either receiving an input signal from an external source, or as a broadband output structure for deriving output energy from an electron stream, or for both of these applications.

For many years one of the limitations in the design of high power electron discharge devices of the velocity modulation type, such as the klystron, has been the unavailability of satisfactory input and output structures capable of both broadband and high power operation. For example, in the past a klystron has been considered inherently a narrow band device because it involves the use of very sharply resonant high-impedance tuned circuits. The necessity for a klystron to utilize such a high impedance tuned circuit for obtaining eflicient interaction therein arises from the fact that the exchange of energy which occurs at the interaction gap between the electron stream and the electric fields takes place over only a very short physical distance.

Because of .the foregoing limitation of the prior art klystron cavity resonators, considerable effort has been expanded in attempts to provide broadband circuit elements, for use in both the input and output circuits of klystrons, which would be capable of being tuned over the requisite bandwidth. Thus far, however, these efforts have met with little success and there still remains the real problem of providing means for tuning the klystron through the use of means which are accurate, simple and rapidly variable. Since no broadband input and output circuits for klystrons have resulted from the efiorts expended in the past, the users of klystrons have been forced to use tuning circuits which require the use of mechanical tuners. These tuners alone have not proven adequate owning to the fact that they tend to limit the bandwidth, do not provide sufficiently precise frequency control, and require time consuming processes for optimum frequency setting.

The present invention obviates the foregoing and other disadvantages of the prior art and solves long standing probelms of the prior art klystron amplifiers. In accordance with the basic concept of the present invention there is disclosed herein a broadband structure of high interaction impedance, which, depending upon the application of the structure, may be modified slightly to operate either as an input structure receptive of electromagnetic energy from an associated input circuit, or as an output structure for transmission of electromagnetic energy from the klystron to an associated output load circuit.

More specifically, in accordance with the present invention, the communication of an input signal to the klystron or the extraction of an output signal therefrom may be accomplished by the use of a device which includes a structure equivalent to a folded ridged wave guide and means shortcircuiting one end thereof, the other end being used for receiving an input signal or the presenting of an output signal to an external load. When the device is employed as an output device, a conventional waveguide member and output window are associated therewith, whereas when the device is employed as the input structure a conventional short-circuited waveguide member and a coupling antenna member are associated therewith. In the preferred embodiment of the invention the folded ridged waveguide is formed by several associated elements which provide a substantially U- shaped structure separated by a common wall, the two legs of the U constituting a pair of mechanically parallel waveguides separated by an interdigital member which divides the spacing between the associated pair of drift tubes into a pair of high impedance interaction gaps for the broadband structure. In accordance with the preferred embodiment of the invention the legs of the U- shaped structure and the interdigital member are apertured concentrically with the pair of drift tubes which terminate respectively adjacent the opposing remote broad walls of the parallel Waveguide to provide a pair of sequential interaction gaps on either side of the interdigital member.

The aforesaid interdigital member associated with the broadband structure will be referred to hereinafter as the interdigital finger may be defined as a flat metallic element having essentially three portions, which may be visualized as an apertured center region and a pair of flat end regions of substantially equal lengths. In accordance with the preferred embodiment of the invention, the center region has a round washer-like shape while the two end regions are slightly narrower in width than the diameter of the washer and are affixed to the washer diametrically opposite one another along a center line cross a diameter of the washer. The diameter of the over a Wide frequency range without the necessity of mechanical tuners.

Another objectof the inventon is to provide a klystron which employs an input or output circuit element or both of these elements having high interaction impedance capable of providing broadband operation without the use of mechanical tuning means.

A further object of the invention is to provide a folded ridged waveguide structure having high interaction impedance capable of functioning as the input or output circuit element for a multi-cavity klystron.

Still another object of this invention is to provide a folded ridged waveguide structure and an associated interdigital member providing means for communicating microwave energy into klystron or extracting microwave energy therefrom over a wide frequency range.

The novelfeatures which are believed to be characteristic of the invention, both as .to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which several embodiments of the invention are illustrated by Way of example. It is to be expressly understood, however, that the drawings are intended for the purpose of illustration and description only, and are'not intended as a definition of the limits of the invention.

FIGURE 1 is an elevation vew, partly in section, of a klystron vacuum tube illustrating the manner in which the broadband folded riged waveguard structure of the invention may be employed as an output structure for a klystron to provide broadband operation at high powers;

FIGURE 2 is a cross-section view of the broadband folded ridged waveguide structure shown in FIGURE 1, illustrating the manner in which the various elements thereof cooperate to provide an output circuit'for extracting energy from the klystron;

FIGURE 3 is a cross-section view of the broadband folded ridged waveguide structure, taken along line 33 of FEGURE 2, illustrating the space relationship and arrangement of the principal circuit elements of the invention;

FIGURE 4 is a cross-section view of the broadband folded ridged waveguide structure, taken along line 4-4 of FIGURE 2, showing the general configuration of the interdigital finger along its length and width, and the location of the aperture therein; and

FIGURE 5 is a view, partly in section, of a klystron vacuum tube illustrating the manner in which the broadband folded ridged waveguide structure of the invention may be employed to provide both input and output structures for the tube.

Referring now to FIGURE 1, wherein the same reference characters designate like or corresponding parts throughout the several views, there is shown a broadband klystron vacuum tube 16 which employs a broadband ridged slow-wave output structure 12, constructed in accordance with the teachings of the present invention, for extracting energy from the tube and presenting an output signal to an external load. As shown in FIGURE 1, the klystron funther includes an input cavity resonator 14 which is disposed adjacent a focusing electrode 16 aligned along the axis of the tube designating a path 18 of an electron stream emitted by an associated cathode 19. The input resonator includes a tuner element 23 to provide means for varying the resonant frequency thereof, and an input circuit element 22 afiixed to the cylindrical wall of the cavity to provide means for in-troduca ing the input signal to be amplified. Continuing, a first drift tube 26 is affixed at one end to the upper end wall of input cavity 14 and at the other end to a second resonant circuit, generally designated as buncher cavity resonator 28. The buncher resonator in turn includes a second tuner element 30 providing means for varying the resonant frequency thereof, and is connected to one of the end walls of the broadband ridged slow-wave output structure 12 through a second drift tube 32. As shown in the drawing there is associated with structure 12- a waveguide output member 38 including a vacuum tight hemispherical output window fabricated from a low loss glass or ceramic. The klystron is completed by a third drift tube 34 which is afiixed at one end to the other end wall of the slow-wave structure 12 for transmitting the electron stream from the slow-wave structure to a collector electrode 36, the collector electrode 36 thus being the terminal element of the device and providing means for collecting the electrons of the electron stream and dissipating the energy generated thereby.

In operation (the electron stream is emitted from the cathode and is focused along the path 18 through the use of the focusing electrode 16 and a solenoidal magnetic field produced by an associated magnet, not shown. The input signal to be amplified is applied to input terminal 22 in the customary manner to establish initial electric fields within the cavity whereby the electron stream is velocity modulated by the fields as it passes through the gap of the cavity 14. The modulated electrons then leave the input cavity and pass into the drift tube 26 enroute to the second cavity 28.

As the electrons arrive at the gap of the buncher cavity 28 they are further modulated by the :gap fields of the cavity, and thereafter pass into the second drift tube 32 where they form more compact bunches and arrive at the first gap of the slow-wave structure '12 in substantially tight bunches. The bunched electrons then interact with the fields of the two gaps of the slow-wave structure '12 to excite or launch an output Wave which is transmitted to an external load through the associated Waveguide 38 and output window 40'.

With reference now to FIGURES 2, 3 and 4, there is shown with greater particularlity several views of the broadband ridged slow-wave output structure shown in FIGURE 1 illustrating the relative position of the elements thereof. Basically the broadband ridged slow-wave structure as shown in FIGURE 2 comprises a cylindrical member 70 which is apertured at opposite ends to admit drift tubes 32 and 34, a raised metallic ridge generally designated 72 which extends diametrically around the interior of cylinder 70 terminating at one end in contact with a short circuiting element 74 and at the other end contiguous with a broad wall of waveguide 38 through an output aperture 75 in the side of cylinder 70, and an apertured interdigital finger 76 which is aflixed to the top of short circuiting element 74 and extends diametrically across the cylinder with its aperture concentric with the drift tubes to divide to gap between the drift tubes into a pair of successive or sequential gaps as viewed by the electron stream.

It should be noted at this point that for ease in fabrication cylindrical member 70 is preferably formed as a pair of opposing cup-shaped elements 78, and 80 into which the remaining elements of the slow-wave structure are afiixed, after which the cup-sl1aped members are joined. Furthermore it should be noted that the metallic ridge 72 is actually formed from four individual segments 81 through 84 respectively, and that drift tubes 32 and 34,

intersect and extend through the ridge and terminate in a pair of gap nose pieces 86 and 88 which are raised above the surface of the segments forming the ridge. Finally, it will be seen that the height of segment 84 forming part of ridge 72 is higher than that of the remaining segments in order to provide an impedance transformation between the impedance of the slow wave output structure and the output waveguide 38.

Consider now the space relationship of the various elements of the entire broadband structure as shown in FIG- URE 2. The drift tubes 32 and 34 have substantially equal internal diameters, and are aligned axially along the path 18 of the electron stream. The gap nose pieces 86 and 88 are disposed opposite one another with a predetermined distance therebetween functioning as parts of the two interaction gaps of the broadband structure. The interdigital finger 76 is interposed an equal distance between these nose pieces with its aperture aligned concentrically with the two drift tubes, the aperture in the finger having a diameter equal to or slightly larger than the internal diameter of the drift tubes. considered as forming a pair of parallel waveguide sections separated by a common wall which is provided by the interdigital finger 76, means interconnecting one end of the first waveguide with the corresponding end of the second waveguide, means short circuiting the other'cnd of the second waveguide, and means forming conductive ridge extending along the remote walls of both waveguides and across the intercommunicating ends of the waveguides.

Continuing, it will he noted further from FIGURE 2 that the physical dimensions of the broadband structure are such that the electrical distance from point A in the first gap along path 18 to the adjacent wall of short circuiting element 74 is an odd multiple of a quarter wavelength at the center frequency of the operating range of the tube, or a minimal distance of Ag/4. The distance from point A to point B in the second gap along path 18, on-the other hand, is an odd multiple of one half of a wave-length at the center frequency, or a minimal distance of Ag/Z, this distance being taken in the plane of the conductive ridge 72.

Referring now to FIGURE 3 there is shown a crosssection view of the slow-wave structure, taken along line 3-3 of FIGURE- 2, illustrating the space relationship and arrangement of the ridge waveguide segments 81, 82, 83 and 84, the gap noses 86 and 88 and the interdigital finger 76. It will be seen that the widths of the ridged waveguide segments are only slightly wider than thetip The composite assembly thus may be V of the nose pieces as and 88, and as such define a path which confines the traveling waves of the device to a volume which is subjected to the maxmum concentration of the field lines between the gaps. As further shown in FIG. 4, which is a cross-section view of the slow-wave structure taken along line 4-4 of FIGURE 2, both ends of the interdigital finger 76 have substantially the same width as the ridge waveguide segments, as may be noted by comparing member 82 and the free end 90 of the finger 76. In practice it may be shown that the traveling waves of the device are confined substantially to this restricted width providing the greatest interaction between the fields produced in the gaps and the waves induced in the folded ridged waveguide.

Consider now the electrical operation of the ridged Waveguide slow-wave structure of the invention, wherein a velocity modulated stream of electrons travel through the drift tube 32 along the axis 18 of the klystron' and arrive at the point designated A within the gap between the gap nose 38 and the interdigit-al linger 76 in tightly bunched groups, where they interact with the electric fields of the gap. During their interaction with the gap fields the bunched electrons deliver output energy by setting up electromagnetic Waves, having associated RF current and voltage components, which divide equally, propagating /2 to the right and /2 to the left of the gap nose.

That portion of the wave which propagates to the right of gap A encounters the short circuiting element 74 which is substantially an odd multiple of quarter wavelength from the center of the gap A. Consequently after the wave impinges upon the wall it is reflected therefrom with a phase shift of 180 electrical degrees and travels to the point designated point B between the gap nose 86 and the finger 76 which is substantially three-quarter wave length from the short circuiting element 74. Owing to the electrical length selected for this path the wave is now in the proper phase relationship with respect to the electron stream so that one period later it combines with the wave being generated and reinforces the fields at point 13 within the gap providing maximum fields for interaction with the bunched electron. The phase relationship between the wave initiated at gap A which travels to the short circuiting element 74 and then returns traveling around the finger, causes greater fields to exist at point B than would normally be present in the gap at the instant the electron bunch arrives at point B. The presence of greater fields at the gap thus creates greater interaction between the fields and the electrons, thereby generating more energy than would be generated in absence of this relationship.

The other wave traveling to the left from gap A around the free end of the finger 76 in turn arrives at the point B in the second gap after traveling a distance approximately equal to an electric one-half wave length, one-half period later and continues through the gap B where it is comunicated to an external load through the output aperture 75.

It should be noted that the phase relationship between the electron bunches and the electromagnetic waves is possible because the transit time of the electron bunches from point A to point B along the tubes axis is very short as compared with the electrical time delay of a wave traveling from gap A to gap B around the end of the finger 76. As will be recognized by those skilled in the art, the velocity of the electron beam is a function of the accelerating voltage between the cathode and its associated focusing electrode 16, and may be controlled by controlling this potential. In addition, the distance between the gap nose 86 and the finger 76, the distance between the gap nose 8S and the finger 76 and the thickness of the finger 76 is so small that the interaction at point A may be considered as occurring substantially at the same time as the interaction at point B.

Considering next the electron stream as it interacts with the fields at point B of the second gap, it will be recognized that another energy wave will be propagated to the left back around the free end of the finger 76 and is reflected from the short circuiting element 74 with a 180 electrical degrees phase shift arriving at point A in proper phase relationship with the incoming modulated electron beam such that it reinforces the fields at point A of the first gap for maximum interaction. As described her-einabove, the-selection of the length of the wave path around the end of the finger 76, three-quarter length to the short circuiting element 74 and another one-quarter wave length after reflection, is such that the proper phase relationship is established with respect to the incoming electron stream in gap A, a period l-ater' permits the wave initiated by the prior electron bunch, which interacted within gap B to reinforce the fields at point A, thereby accounting for the same type of aforesaid maximum interaction at point A as there is at point B.

It will be recognized from the foregoing discussion that the cycle described hereinabove is then repeated each time a modulated bunch of electrons enter the slow-wave structure. It should be noted that the wave propagated to the left from point B is conventionally considered a backward Wave and reinforces the fields at point A because of the previously described phase relations which exist between the grouped electrons and the traveling waves. Furthermore, it will be understood that simultaneously there is another wave propagated to the right of point B which combines with the wave received from gap A and is transmitted to an external load through the use of matching impedance transformer 88, the output Waveguide 38 and the output window 40.

Although the foregoing description relates to the use of slow-wave folded ridged wave-guide structure of the invention for extracting the output signal from a multi cavity klystron, it should be emphasized that the basic concepts herein set forth may also be employed for receiving the input signal in a multi-cavity klystron. With reference to FIGURE 5, for example, there is shown a klystron which employs both a broadband rigid slowwave input structure and the broadband ridged slowwave output structure lz discussed hereinabove, the input structure being used for receiving an input signal from an external source in accordance with the teachings of the present invention. There is associated with this particular embodiment of the invention a buncher section 10-2 which includes a series of stagger-tuned cavity resonators, the details of which are not shown, to provide operation over a relatively wide frequency range without the necessity of mechanically tuning the cavities.

The ridged slow-wave input structure 100 may be constructed in a manner similar to that of the slowwave structure 12 shown in FIGURES 1 through 4 except that the output window previously described is replaced with an input coaxial feed 104 and a short circuit wall termination 106 associated therewith. The Waveguide 58 is again a conventional rectangular wave guide having the short circuited termination 106 positioned with respect to an antenna probe 108 in such a manner as to transmit to the input slow-wave structure the input signal applied to coaxial input 104. In operation the input ridged waveguide functions inversely to the manner previously described in that it provides a double gap for velocity modulating the electron beam by virtue of the interdigital finger 110, while simultaneously exhibiting broadband characteristics. It should be noted that while the coaxial input arrangement shown in FIG- URE 5 should be found to operate satisfactorily, it may be considered advantageous toemploy a suitable one way microwave transmission device, such as a microwave ferrite isolator, in conjunction with the input line. Utilization of such a device will substantially minimize or eliminate any tendencies the ridged slow-wave input structure 1% may have to oscillate because of reflections generated within the system.

Consider now the advantages which are derived through the use of the novel broadband slow-wave structures constructed in accordance with the teachings of the invention as described hereinabove. First, it will be recalled that the internal configuration of the slow-Wave structure includes two gap nose pieces 76 and 78, and the interdigital finger 80 which function to determine certain important characteristics thereof. These characteristics are the inductance of the structure, which is substantially determined by the size and shape of the interdigital linger 80, while the capacit-anm of the structure is determined by the spacing between the gap noses 76 and 78, and the finger 80. Thus, when it is necessary to predict the interaction impedance of the structure, which is a function of its capacitance and inductance, it is possible to do so through the use of known mathematical techniques by varying the sizes and shapes of the finger 80, and the gap noses 76 and 78, and by varying the spacing between the gap noses and the finger.

A second advantage arises from the fact that there is provided a ridged waveguide structure which is inherently broadband and which functions to concentrate the electric fields in the space surrounding the interaction gaps to provide maximum field for the maximum interaction impedance which is required for high power operation. Moreover, the ridged waveguide structure also functions as an impedance transformer to match the slow-wave structure to its associated external circuit providing operation over a wide range of frequency without the necessity of mechanical tuning. Finally, it should be noted that the structure of the invention distinguishes over the prior art traveling wave tube structures in that in conventional traveling wave tube amplifiers the backward traveling wave, Where a backward traveling wave is defined as one which travels in a direction opposite to the forward direction of the electron beam, is normally propagated backwards down the slow-wave structure opposite to the flow of electrons without contributing -to the power output of the slow-wave structure. In the present in-' vention, on the other hand, the presence of the short circuit wall 74 permits this usually non-contributing wave to be used to advantage by virtue of its reflection back to the interaction gap in the proper phase relationship where it reinforces the attendant fields providing greater interaction at the gap.

Heretofore, no known high power klystron tube has existed which would operate over a range of frequency substantially in excess of 5% of the operating frequency range without the necessity of tuning the input, buncher and output structures from frequency to frequency over the desired frequency range. In contrast the klystron of the present invention, in one preferred embodiment, eliminates the necessity for mechanically tuning the output structure. In the other preferred embodiment, in turn, the necessity for tuning either the input, buncher, or output structures is eliminated, and the device is capable of operating over a wide range of frequency, for example 20% bandwidth. It has been found that the utilization of the basic concepts set forth herein provides slow-wave structures which are capable of operating over frequency ranges and power levels greatly exceeding what has heretofore been achieved by any known prior art structures in combination in a klystron. By way of example pulsed k-lys-tron tubes have been produced in the 1200 to 1500 megacycle frequency range which operate over a frequency range of more than several hundred megacycles at megawatt levels without the necessity of tuning the input or the output structures.

While the slow-wave structure of the invention has been described with reference to only two particular embodiments, it will be understood that various modifications could be made in the construction thereof without departing from the spirit and scope of the invention.

For example, the free ends of the drift tubes, which have a configuration like the frustu-m of a'cone and function i as gap nose pieces, may be omitted such that the drift tubes terminate substantially flush with the interior Walls of the structure. The omitted cones may be replaced by a pair of cones similar to those omitted, being disposed on opposite sides of the interdigital finger and afiixed thereto in alignment with the openings in the drift tubes to provide a pair of interaction gaps which would be physically and functionally similar to those discussed hereinabove. As a second example, the interdigital finger could be modified by expanding the width of the linger, on both sides of a center line laying along the center of the length of the finger, such that the expanded finger intersects the cylindrical walls of the lower cup-shaped element which forms one half of the enclosure for the structure. The expanded member would thus form a chordal plate whose unengaged straight edge is defined by a chord drawn perpendicular to the center line through the original finger and intersecting the two points on the circumference of the cup-shaped member determined by the intersecting plate. The opening between the end of the plates edge, and the associated ridge and cup-shape element would still permit the waves induced in gap A to travel around the edge of the plate to gap B and vice versa in a similar fashion as disclosed in the preferred embodiment of the invention. As a third example, the structure disclosed herein could be modified such that it would operate as a broadband buncher cavity resonator having two interaction gaps. In order to accomplish this, the pair of opposing cup-shaped elements would be made to form a cylindrical enclosure having resonant characteristics, and would have the output end of the structure terminated by a loading means such as a lossy ceramic material instead of terminating the output end with an output waveguide section and an output window as dis closed in the present invention. In substantially all other respects the structure will be the same, that is, the structure will have two nose pieces and an interdigital finger [forming two interaction gaps, a plurality of ridge members and a shorting wall at one end of the slow wave structure. ture will cause it :to have a Q lower than that of a similar structure without the loading, thereby providing a structure capable of broadband operation. Accordingly, it is to be expressly understood that the foregoing description shall be interpreted only as illustrative of the invention and that the appended claims be accorded as broad an interpretation as is consistent with the basic concepts herein taught.

What is claimed as new is:

1. In a broadband klystron wherein energy is exchanged between a radio frequency field and a beam of electrons passing through said field, the combination comprising: a folded ridged waveguide forming a pair of physically parallel ridged waveguides interconnected at one end and separated by a common wall, first and second concentrically positioned drift tubes respectively affixed to and extending through the walls of said pair of ridged waveguides remote from said common wall, said drift tubes being spaced from said common wall on opposite sides thereof by a preselected distance, said common wall having afi aperture formed therein with a diameter at least as large as the inside diameter of said drift tubes and located.

concentrically with the axis of said drift tubes; means for projecting an electron beam through said first drift tube and the aperture in said common wall and into said second drift tube, and means short circuiting other end of one of said pair of ridged waveguides.

2. The combination defined in claim 1 which further includes Wave guiding means connected to the other end of the other of said pair of ridged waveguides and forming therewith a microwave transmission system for coupling to the electron beam as it passes through the pair The addition of the lossy material to the strucof gaps defined by said apertured common wall and said first and second drift tubes.

3. In a broadband klystron wherein energy is exchanged between a radio frequency field and a beam of electrons passing through said field, the combination comprising: a folded ridged waveguide forming a pair of physically parallel ridged waveguides interconnected at one end and separated by a common wall, first and second concentrically positioned drift tubes respectively afiixed to and extending through those walls of said pair of ridged waveguides remote from said common wall, said drift tubes being spaced from said common wall on opposite sides thereof by a preselected distance, said common wall having an aperture formed therein in alignment with said drift tubes; means for projecting an electron beam through said first drift tube and the aperture in said common wall and into said second drift tube, and means short circuiting the other end of one of said pair of ridged waveguides, the distance between said short circuiting means and the pair of gaps formed by said drift tubes and said common wall being M4 and 3M4, respectively, at the center frequency of the operating frequency band.

4. In a klystron, the combination comprising: first and second electron beam channeling tubes, said tubes being aligned concentrically with respect to each other and being separated by a gap of preselected length; means for projecting a beam of electrons through said first beam channeling tube, across said gap and into said second beam channeling tube; and energy conversion means for exchanging radio frequency energy with the electron beam, said conversion means including a metallic cylindrical member positioned concentrically with respect to said beam channeling tubes and surrounding the gap between said tubes, said member being afiixed at opposite ends thereof to said first and second beam channeling tubes, means forming a raised ridge in the interior of said cylindrical member extending diametrically across the ends thereof and along the inside wall on at least one side of said cylindrical member, said ridge having a planar configuration and being formed to admit the ends of said first and second beam channeling tubes into said cylindrical member, an interdigital finger afiixed to the side wall of said cylindrical member at a point diametrically opposite the ridge on the side of said cylindrical member, said finger extending outwardly normal to the axis of said beam channeling tubes in the plane of said ridge, said finger being positioned to biseot substantially the gap between said beam channeling tubes and having an aperture therein concentric with said beam channeling tubes to permit passage therethrough of the electron beam, and means forming a short circuit termination between the base of said finger and the adjacent end of the ridge extending diametrically across one end of said cylindrical member, whereby said ridges and said finger provide a broadband microwave transmission line.

5. The combination defined in claim 4 wherein said energy conversion means is employed for extracting microwave power from the electron beam, and wherein said energy conversion means further includes an output aperture formed in said cylindrical member between the base of said finger and the adjacent end of the ridge extending diametrically across the other end of said cylindrical member, and waveguide means connected externally of said member around said output aperture.

6. The combination defined in claim 4 wherein said energy conversion means is employed for receiving a microwave input signal for velocity modulating the electrons in the electron stream, said energy conversion means further including a coaxial input conductor terminated at a point in the side of said cylindrical member between the base of said finger and the adjacent end of the ridge extending diametrically across the other end of said cylindrical member.

7. In a broadband klystron wherein microwave energy is extracted from a velocity modulated electron beam as ill 1% it passes between first and second spaced and concentrically aligned drift tubes, a broadband output structure comprising: a metallic cylindrical member positioned concentrically with respect to said drift tubes and sur rounding the gap between said tubes, said member being aifixed at opposite ends thereof to said first and second tubes; means forming a raised ridge in the interior of said cylindrical member extending diametrically across the ends thereof and along the inside wall on at least one side of said cylindrical member, said ridge having a planar configuration and being formed to admit the ends of said first and second drift tubes into said cylindrical member, an interdigital member afii-xed to the side wall of said cylindrical member at a point diametrically opposite the ridge on the side of said cylindrical member, said finger extending outwardly normal to the axis of said drift tubes in the plane of said ridge, said finger being positioned to bisect substantially the gap between said drift tubes and having an aperture therein concentric with said drift tubes to permit passage therethrough of the electron beam, and means forming a short circuit termination between the base of said finger and the adjacent end of the ridge extending diametrically across one end of said cylindrical member, whereby said ridges and said finger provide a broadband microwave transmission line.

8. In a broadband lclystron wherein output energy is extracted from a velocity modulated electron beam as the beam traverses an opening between first and second concentrically aligned drift tubes, a broadband output transmission system for establishing a radio frequency field in response to the velocity modulated electrons in the beam and for launching a microwave output signal, said system comprising: an evacuated housing member affixed to said first and second drift tubes and surrounding the opening therebetwecn; separator means extending from one side of said housing member toward the opposite side normal to the axis of said drift tubes and equidistant there-betwee said means having an aperture therein aligned with said drift tubes and having an axis of symmetry normal to the axis of said drift tubes whereby the opening between said drift tubes is divided into two successive gaps; means forming a U-shaped ridge around the interior of said housing member in a reference plane intersecting the axis of said drift tubes and said axis of symmetry, said separator means extending between the legs of said U-shaped ridge and being spaced from the base of said ridge; means forming a short circuit between said separator means and the end of one of the legs of said U-shaped ridge, and waveguide means afiixed to said housing member for extracting output energy from a point between the base of said separator means and the end of the other of the legs of said U-shaped ridge, said short circuiting means being spaced from the axis of said dnift tubes :by a distance substantially equal to A/ 4 Where A is the wavelength of the center frequency of the frequency range over which the tube is operablefand said sepanator means extending across said housing member to a point where the electrical distance from the axis of said drift tube in one of said gaps to the same axis in the other of said gaps around the end of said separator means is substantially equal to A/ 2 at said center frequency.

9. In a klystron, the combination comprising: a oathode and an electrostatic focusing means for producing and directing an electron stream of a predetermined diameter along a predetermined axis; means forming a first modulating gap for modulating the velocity of electrons in said stream in accordance with an applied microwave input signal; first drift tu be means for allowing said modulated electrons to form groups in said stream; means forming a second modulating gap and an associated resonant circuit responsive to the grouped electrons :for further modulating said electron stream; second drift tube means for allowing said modulated electrons to form more compacted groups in said stream; collector means concentric with and spaced from said second drift tube means for collecting the stream of electrons; and a broadband slow wave output structure for extracting electrical energy from said electron stream as the groups of electrons traverse the space between said second drift tube and collector means and for transmitting electromagnetic energy waves to an external load, said output structure comprising a pair of conjugate circular cup members connected to each other along their circumferential edges and atfixed respectively at their ends around said second drift tube means and said collector means to provide an evacuated toroidal enclosure, a plurality of ridged wave guide members and a pair of gap noses connected together in series within said enclosure to form a U-shaped structure having two parallel legs, one of which is shorter than the other, said gap noses each having a circular aperture therein for terminating said second drift tube and collector means and being disposed opposite one another with said apertures in axial alignment, a short circuit member Iafiixed to the end of said shorter leg for reflecting eleotro-magnetic Waves which impinge thereon, an interdigital finger having a circular aperture therein which is affixed to said short circuit member and disposed parallel to said legs and equidistant therebetween having said aperture of said finger in axial alignment with that of said gap noses, an output waveguide having one end connected externally to the cylindrical wall of said toroidal enclosure and disposed perpendicular to said axis and (adjacent the longer leg of said U-shaped structure, and an output window heremetically sealed to the other end of said waveguide for passing electromagnetic energy to an external load.

10. In a klystron, the combination comprising: a cathode and an electrostatic focusing means for producing and directing an electron stream of a predetermined diameter along a predetermined axis; a broadband slow-wave input structure defining a pair of modulating gaps for modulating the velocity of electrons in said stream in accordance with an applied microwave input signal said input structure comprising a pair of conjugate circular cup members connected to each other along their circumferential edges providing an evacuated toroidal enclosure, a plurality of ridged wave guide members and a pair of gap noses connected together in series Within said enclosure to form a U-shaped structure having two parallel legs, one of which is shorter than the other, said gap noses each having a circular aperture therein dis posed opposite one another with said apertures in axial alignment, a short-circuited member afiixed to the end of said shorter leg for reflecting electromagnetic waves which impinge thereon, an interdigital finger having a in said input structure for receiving the electron stream after it has passed therethrough to permit the. velocity modulated electrons to form groups in said stream; and means forming a plurality of modulating gaps aligned along said axis in a position for receiving and passing said electron stream in succession and for extracting electrical energy from said electron stream.

11. An electron discharge tube of the velocity modulation type, said tube comprising means for producing a beam of electrons along a predetermined axis of said tube, means for velocity modulating said beam, an electrode for receiving electrons in said beam, drift tube means surrounding the path of said electron beam after said beam has been modulated and a ridged slow-wave structure for extracting energy from said beam, said strucuire comprising first and second gap noses disposed opposite one another along the axis and forming a-portion of the means surrounding the path of the beam, means disposed at M4 from said axis of said first gap nose for reflecting a traveling wave, a slow-Wave finger interposed between and equidistant from said gap noses,

a ridged Waveguiding element extending from said first gap nose around the tip of said finger to said second gap nose, an impedance transformer adjacent said second gap nose providing means for matching said structure to an external load, and an out-put wave-guide aflixed adjacent said transformer external to said slow-wave structure, whereby energy generated within the tube may be extracted therefrom.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN A BROADBAND KLYSTRON WHEREIN ENERGY IS EXCHANGED BETWEEN A RADIO FREQUENCY FIELD AND A BEAM OF ELECTRONS PASSING THROUGH SAID FIELD, THE COMBINATION COMPRISING: A FOLDED RIDGED WAVEGUIDE FORMING A PAIR OF PHYSICALLY PARALLEL RIDGED WAVEGUIDES INTERCONNECTED AT ONE END AND SEPARATED BY A COMMON WALL, FIRST AND SECOND CONCENTRICALLY POSITIONED DRIFT TUBES RESPECTIVELY AFFIXED TO AND EXTENDING THROUGH THE WALLS OF SAID PAIR OF RIDGED WAVEGUIDES REMOTE FROM SAID COMMON WALL, SAID DRIFT TUBES BEING SPACED FROM SAID COMMON WALL ON OPPOSITE SIDES THEREOF BY A PRESELECTED DISTANCE, SAID COMMON WALL HAV- 